Stem cell (HSC) self-renewal and differentiation are central to the maintenance of normal hematopoiesis. During the previous grant period, we performed an extensive analysis of HSC on the clonal level. The data show that HSC behaviors are largely fixed on the level of the HSC. Consequently, the adult HSC compartment consists of subsets of HSC, each with preprogrammed self-renewal and differentiation capacity. We showed that the HSC compartment can be divided into 3 subsets, namely balanced, myeloid-biased, and lymphoid-biased HSC. Lineage-biased HSC share with balanced HSC the following features; 1) They contribute to all hematopoietic lineages, albeit with a noticeably skewed ratio; 2) Lineage bias is a stable HSC intrinsic feature that is inherited to daughter HSC; 3) All types of HSC can have extensive self-renewal capacity, but My-bi HSC as a population have more self-renewal capacity than other types of HSC. The discovery of these HSC classes now opens the unique opportunity to define the developmental and molecular mechanisms that control lineage potential and self-renewal capacity on the level of the HSC. We propose to define the onset of the fixed behavior during development, define whether epigenetic events contribute to lineage-bias, and test whether myeloid-bias predisposes HSC for myeloid Leukemia. Currently, much effort is spent on trying to change HSC to improve their performance in transplantation and therapeutic applications. However, the finding that the HSC compartment consists of distinct classes of HSC with fixed behavior predicts that attempts to modulate the behavior of HSC will have limited success. Rather, different types of HSC could be selected to be optimal for each application. To find the right HSC for each application, we need to better understand the molecular and developmental mechanisms that distinguish the different types of HSC. Relevance: Data from the proposed studies may provide better ways to treat leukopenia and may help in the design of approaches to selectively eradicate leukemic stem cells.